1. Field of the Invention
The present invention relates to a method for efficiently isolating and purifying crystals, especially of amino acids, nucleic acids or derivatives thereof from solutions containing both crystals and bacterial cells.
2. Discussion of Background
As methods for separating crystals in a culture broth or an enzyme reaction solution, filtration or centrifugal precipitation has heretofore been used generally. Where cells are contained in the solution to be treated, however, the cells tend to cause clotting of the filter cloth during filtration and most of the cells are taken up into the crystals so that filtration is not suited for separation of crystals. Using centrifugal precipitation, represented by the decanter type, it is possible to isolate crystals but selection of cells and adhered mother liquor is insufficient. It is thus necessary to perform multiple step separations and additional countercurrent washing where loss in separation of crystals is unavoidable. In another method, an acid or a base is added to dissolve the crystals, and after removing the cells from the solution, recrystallization must be performed.
A liquid cyclone is well known as a wet type classifying machine which gives a centrifugal force in a fixed cylinder by rotating liquid and has been widely used for recovery of finely ground matter in a mine, removal of sand grains from pulp, classification of various starches, etc. These are all due to application of the function of a liquid cyclone to classify particles having a diameter larger than the critical diameter into the downstream side. In this case, however, a liquid cyclone fails to classify particles having a diameter smaller than the critical diameter and these particles are merely classified by a ratio of liquid volume at the upstream to that at the downstream. This concept is conventionally applied to a crystal slurry of a solution containing crystals to be concentrated using a liquid cyclone. But, when a solution containing cells is fed to the liquid cyclone, the cells have a diameter, smaller than the critical diameter so that there is no difference in concentration between the upstream and the downstream of the liquid cyclone, which makes separation of the cells impossible. Therefore, there is a need for a method to efficiently isolate crystals from a solution containing cells using fewer steps and at low cost.